The invention relates to an extrusion device for influencing the wall thickness of an extruded plastic profile and a method for influencing the wall thickness of an extruded plastic profile.
Plastic profiles are often produced by extrusion. A mostly homogeneous plastic melt is prepared in an extruder and brought to a pressure of about 200 to 400 bar and a temperature of about 200° C. The plastic melt is pressed with a high pressure through an extrusion nozzle. The outlet of the extrusion nozzle has approximately the shape of the desired plastic profile.
After the plastic melt leaves the extrusion nozzle, the extruded plastic profile, for instance in form of a melting strand, enters a calibration device in order to be cooled down in said device by maintaining the profile shape. Calibration devices for the production of comparatively complicated plastic profiles, as for instance window profiles as hollow profiles, comprise usually a dry calibration area and a subsequent wet calibration area.
The wall thicknesses of the plastic profile are thereby determined largely in the nozzle.
A change of the wall thicknesses in the calibration is almost not possible. In case of hollow chamber profiles the corresponding outer wall is sucked to the calibration and is then cooled mainly via heat dissipation. No influence which could provide a change of the wall thicknesses can be exerted onto the inner side of said outer wall (i.e. within the plastic profile).
In case of, for instance single walled, profile sections projecting from the plastic profile, as for instance hooks or noses, a pressure force could be exerted on both sides in the calibration, which would displace material sideways and would thus provide a lower wall thickness. Practically, this is only possible to a very limited extend, since a clamping force required thereto would inevitably provide a friction force, which cannot be transferred by the still soft, viscous plastic mass. The affected profile section would get stuck in the gap being too narrow and would lead to a breakaway of the profile strand.
The gap or the shape in the calibration has thus always to be larger for shaping single walled sections than the corresponding wall thickness or shape of the plastic profile in order to avoid reliably that the extruded plastic profile gets caught in the calibration, also if due to slightly, unavoidable deviations of the throughput from the extruder corresponding deviations of the wall thicknesses are transferred onto to the plastic profile. It is thus common to realize the gap in the calibration about 0.1 to 0.3 mm, in particular 0.2 mm larger than it corresponds to the wall thickness of the profile.
When producing extrusion devices a substantial construction effort in the extrusion nozzle is required in order to guarantee the wall thicknesses in all profile sections of the extruded plastic profile. This effort relates to changes of the gap width in the flow channels of the extrusion nozzles along the flow path.
The wall thicknesses of the plastic profile are only reproducible as long as the extrusion conditions do not change. The extrusion conditions relate to all parameters in the extruder (extruder type and size, temperature course in the cylinder, rotation speed of the screw, throughput and such) as well as the material (formulation, mixing conditions, moisture content and others).
Each change of the mentioned parameters can cause a change of wall thicknesses at specific sections of the profile, i.e. the whole plastic profile does not tend to become thicker or thinner, but the effects are sectionally different.
A requirement from an economical perspective is to produce plastic profiles with a meter weight as low as possible and to fulfil simultaneously all quality requirements (geometry, minimal wall thicknesses, mechanical stability, gloss and such). It is for instance not possible to provide outer walls being sectionally thinner than determined in relevant standards. The unavoidable interactions during the conventional extrusion have to be considered: If a profile is produced close to the minimal meter weight with good quality this means that each profile section comprises approximately the lower permitted wall thickness.
If due to any disturbance in a profile section, for instance at a hook, the minimal wall thickness is fallen below, a further negative shaping of the local geometry (for instance “hanging” of an edge or an inaccurate shaping of the undercut of the hook) is often also provided.
In order to transfer this one profile section into the right quality, it is a common measure either to increase the throughput or to reduce the withdrawal velocity. Each of both measures provides a local thickening of the wall thickness in the respective profile section, but also inevitable to a thickening of all other profile sections. After all a plastic profile with the corresponding quality is produced, however a higher material input is required.